The present invention relates to a particle counter (hereinafter referred to as a "grain" counter) such as, for example, a blood corpuscle counter for measuring the size of a blood corpuscle and the number of blood corpuscles, thereby examining the blood and, more particularly, to a grain counter which makes it possible to obtain a fine, stable blood column.
In a blood corpuscle counter, in order to increase the measuring precision, it is necessary to cause diluted blood to flow into a pipe having a small inner diameter and make the region to be measured by a sensor small. However, blood to be measured sometimes contains dust, which is introduced when it is extracted, or coagulated components. When such blood is allowed to flow through the fine pipe, the interior of the pipe is blocked. This is one of the drawbacks inherent in this type of blood corpuscle counter.
A water sheath technique has been proposed for eliminating this drawback. In FIG. 1, a type view showing the water sheath principle is shown. A flow cell 10 has an outer tube 12 and an inner tube 14, for example, concentrically disposed in the outer tube 12. A sample solution 18 such as, for example, diluted blood is allowed to flow through the inner tube 14 in a direction indicated by a solid line. On the other hand, a sheath solution 16 such as, for example, physiological saline solution is allowed to flow through the interspace between the outer tube 12 and the inner tube 14 in a direction indicated by a broken line. At a terminal end of the inner tube 14, two flows of solution, consisting respectively of a solution column of sample solution 18 and the sheath solution enclosing this solution column, are created. By reducing the inner diameter of the outer tube 12 having no inner tube 14 disposed therein, the diameter of the two-flow unit consisting of the sheath solution 16 and the sample solution 18 is reduced, whereby a water sheath is formed wherein a fine flow of sample solution 18 is sheathed by the sheath solution 16. In this case, the inner diameter of the inner tube 14 through which the sample solution is allowed to flow can be made large enough to prevent the interior of the inner tube from being blocked by the sample solution 18. On the other hand, even when the cross sectional column of sample solution is reduced by the small-diameter portion 13 of the outer tube 12, this small-diameter portion can not be blocked by the sample solution 18 since the sheath solution 16 exists around the column of sample solution.
By the way, in a conventional blood corpuscle the water sheath is formed with the use of the device shown in FIG. 2. The outer tube 12 is connected to a closed talk or reservoir 20 by means of a pipe 22. On the other hand, the inner tube 14 is connected to a closed reservoir 24 by means of a pipe 26, The reservoirs 20 and 24 are connected to a compressed air supply source by means of pipes 28 and 30, respectively so that compressed air may be supplied to the reservoirs 20 and 24, respectively. By supplying two sources of compressed air with pressures of P1 and P2 to the closed reservoirs 20 and 24, respectively, the sheath solution 16 and sample solution 18 are supplied to the outer tube 12 and the inner tube 14 through the pipes 22 and 26, respectively.
In this type of device, however, it is necessary to precisely adjust the difference between the pressures P1 and P2 of the compressed air, used to deliver the sheath solution 16 and the sample solution 18, to a specified value. Further, where water impurities, proteins in the blood, or the like are adhered onto the inner wall surface of the pipes 22 and 26, whereby the flow resistance in the pipes 22 and 26 varies, the column of sample solution of the water sheath is varied in diameter, or in the worst case the water sheath is not formed. This constitutes one of the drawbacks inherent in the conventional device.